1. Field of the Invention
This invention relates to a thermally insulated flask primarily intended for use outdoors, and made of two containers, where one of the containers is placed inside the other. The outer container is preferably made of aluminum and the inner container forms a neck at the top, which inner container is connected at the top with the outer container with a material of poor thermal conductivity in an airtight and rigid manner. This invention further relates to a process for producing such a thermally insulated flask.
2. Description of Prior Art
Thermos flasks are known which include a flask of a material with good thermal insulating properties for receiving the liquid, or made of double-walled glass, as well as a jacket of plastic enclosing the flask for protection and aesthetics.
A satisfactory insulating effect is achieved with double-walled flasks of chrome-nickel steel, because chrome-nickel steel has a low thermal conductivity coefficient of approximately 14 W/(mK) at 20.degree. C. For production reasons it is necessary to weld the individual chrome-nickel steel flasks together lengthwise. Two such flasks, one placed inside the other at a small distance, are welded together at the top of the flask. The evacuation of the air or the gas between the two flasks results in a considerable improvement of the insulating capability. Flasks with a double wall and a good vacuum in the space between the double walls increase the insulating effect by a factor of 4 to 9, compared with non-evacuated flasks. To obtain a vacuum, the flasks, which are placed one inside the other, are welded together in a vacuum chamber, or, the gas in the hollow chamber is pumped out through a specially installed small tube leading into the chamber, and the small tube is welded shut to be vacuum-tight.
However, such flasks have a number of disadvantages. For one, such flasks are relatively heavy, because chrome-nickel steel has a density of approximately 7.9 g/cm.sup.3, and furthermore, chrome-nickel steel per se is an expensive material. Further the production, which includes welding chrome-nickel steel together, is relatively difficult, elaborate and expensive.
Today, increasingly higher demands are made on thermally insulating flasks. Thermally insulating flasks, which make it possible to maintain a sufficient amount of a liquid dependably at a customary serving temperature for at least some hours, are demanded for the outdoors, for example by mountain bikers, mountain climbers, skiers, hunters and other sportsmen, as well as workers whose jobs take them outdoors. Such flasks furthermore should not be unnecessarily heavy and, of course, should be affordable. These last two requirements in particular are not satisfactorily met by conventional chrome-nickel steel flasks.